The use of stacked configurations of alternating spacing plates and pairs of semi-permeable membranes for diffusing matter between two fluids has been known for many years. Such apparatus are thus disclosed in a number of patents, including Swedish Pat. Nos. 218,441 (U.S. Pat., No. 3,411,630), 301,029 (U.S. Pat. No. 3,501,011), 325, 370 (U.S. Pat. No. 3,511,381), 314,167 (U.S. Pat. No. 3,516,548), and 342,144 (U.S. Pat. No. 3,734,298). In U.S. Pat. No. 3,734,298, for example, such a device for use in dialysis is demonstrated. The device in this patent thus includes means for pressing the alternating spacer plates and pairs of membranes together between a pair of clamping plates utilizing clamping rails extending along and engaging grooves provided along the sides of both clamping plates. As shown in that patent, the clamping plates thus are provided at either end of the stacked configuration, and do not extend laterally below any of the spacing plates and/or pairs of membranes, and the clamping rails shown therein (see FIG. 8) must then be attached by engaging the grooves on either end of the clamping plates. A problem in all of these constructions has been to provide a pressure which is evenly distributed over the entire working surface, as well as providing a suitable pressure in the areas around the through flow passages in order to provide an adequate seal at those points. If this is not obtained, there is a risk that an exterior leakage will occur, and/or a leakage between the two fluids, which is entirely unacceptable.
Reference can also be made to a number of other patents which attempt to demonstrate such diffusion apparatus which include alternating stacks of spacing plates and pairs of semi-permeable membranes, each of which suffers from the deficiencies disclosed above. These include U.S. Pat. Nos. 3,494,469; 3,219,573; 3,051,316; and 3,219,572.